The term ISO describes the sensitivity of the film to light. The higher the number the more sensitive the film becomes. Films usually range from 25 (you would use this in the lightest environment imaginable) to 6400 (to be used at night or in very dark situations). I'm sure you can even get films outside of that range but I have personally never used them. I usually use an ISO 400 film, which is a very good basic film speed that can be used it a broad range of situations. If I know it will be very sunny, I might go to an ISO 100 or 160, but that doesn't happen too often. Similarly I don't often use super sensitive film, like 1600, 3200 or even 6400. The higher the Iso number gets the more grain you will have on your final photograph.
Film can also be pushed (increase sensitivity) or pulled (decrease sensitivity) which results in a very different look. The film will get developed differently and will therefore show more or less grain, according to the process. Exposing film at a different speed than its native one is sometimes useful when I have a film with me that doesn't match the conditions. Here I used the film Ilford Delta 3200 and pushed it to 6400, which also increased the appearance of the grain. This was actually a double exposure that I did with my partner, Johannes, so it might look different if it were only a single exposure.
When thinking about the image I want to create I usually start by selecting a colour or black and white film first and then the next decision will be what ISO I want to use. To me this decision is about balancing the conditions of the environment I will be photographing in with the look I want to achieve. I think this demonstrates kind of nicely how many decisions I am already making before I have even started taking a photograph. It is all about understanding and balancing the different components of photography.